Regulated DC power supplies are typically employed in analog and digital electronic systems. Two major categories of regulated DC power supplies are linear power supplies and switching power supplies. In linear power supplies, a transistor (operating in its active region) is provided in series with a transformer, e.g., a 60 Hz transformer, to provide electrical isolation between an input and an output and to provide the output in a desired voltage range.
In switching power supplies, transformation of a DC voltage from one level to another is accomplished with DC/DC converter circuits, such as step-down (buck) or step-up (boost) circuits. Solid-state devices, such as transistors, are operated as switches (either completely ON or completely OFF) within the switching converters. Since it is not necessary to operate the power devices in their active region, this mode of operation results in lower power dissipation. Furthermore, the increased switching speeds and higher voltage and current ratings of the power devices are some of the factors that have increased the popularity of switching power supplies.
Modern specifications for power supplies generally require operating efficiencies significantly above the specifications acceptable in the past. This is due, in part, to the stringent environments in which modern power supplies operate. The current environments require higher power supply densities and the amount of dissipated heat allowable is limited as compared with past environments. Accordingly, the individual components as well as the overall power supply must be more efficient.
A major source of power loss in switching power supplies is the losses associated with the individual components therein. One such component which is a major source of power dissipation is the semiconductor power switch. Power dissipation in the power switch is generally caused by the simultaneous occurrence of voltage across and current passing through the switch during its turn-off and turn-on transitions. For example, losses occur during the time intervals when the voltage across the switch reduces concurrently with a current flow through the switch. The power dissipation is a direct function of the current level passing through the power switch. With the recent trend of providing higher output currents from the power supplies, this has become an area of concern for power supply designers.
One approach for reducing the turn-on transition dissipation of the power switch is the use of snubber circuits to control the voltage and current of the power switch during switching transitions. The snubber circuits have been used very successfully in high output power supplies of the past. A characteristic of the conventional snubber circuit involves a voltage reduction time interval that is dependent on the amplitude of current flowing through the snubber circuit. Snubber circuits and circuits to control the operation thereof have been the subject of many references. For example, a driver circuit for the power switches and snubber switches of a switching power supply is disclosed in U.S. Pat. No. 5,461,302, issued Oct. 24, 1995, to Garcia, et al. ("Garcia"), which is herein incorporated by reference.
In Garcia, a latch circuit is employed to ensure the complementary switching operation of the power and an active snubber switch in the snubber circuit. The physical characteristics of the switching devices, however, increase the cost and complexity of the switch driver circuit. The drain-source voltage of the power switch has to be driven to zero or a threshold value before the power switch may be turned ON, i.e., conducting. Garcia employs multiple transistors in the latch circuit of the driver circuit to prevent malfunction of the power switch due to the ON resistance, i.e, Rdson, of the device. As a result, a significant voltage (drain-source voltage) may be induced across the power switch which may cause the latch circuit to malfunction. To reduce the risk of malfunction, a higher supply voltage must be employed in the latch circuit which, ultimately, increases the power losses associated therewith. Additionally, the latch circuit employs multiple switches (more specifically, three transistors) that increase the cost and complexity of the switch drive circuit.
Accordingly, what is needed in the art is an improved control system and switch driver circuit that controls the operation of a switch in a power converter that overcomes the deficiencies in the prior art.